Solid high-energy compositions, such as propellants, explosives, gasifiers, or the like, comprise solid particulates, such as fuel particulates and/or oxidizer particulates, dispersed and immobilized throughout a binder matrix comprising an elastomeric polymer.
Conventional solid composite propellant binders utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents. As outlined in detail in U.S. Pat. No. 4,361,526, there are important disadvantages to using cross-linked elastomers as binders. Cross-linked elastomers must be cast within a short period of time after addition of the curative, which time period is known as the "pot life". Disposal of a cast, cross linked propellant composition is difficult, except by burning, which poses environmental problems. Furthermore, current state-of-the-art propellant compositions have serious problems that include, but are not limited to: use of nonenergetic binders, high end-of-mix viscosities, thermally labile urethane linkages, and extreme vulnerability to unscheduled detonation.
Cross-linked elastomers in which prepolymers are polyethers derived from oxetane derivatives and tetrahydrofuran (THF) are described in U.S. Pat. No. 4,483,978, issued to Manser. Urethane curing is achieved with isocyanates and additional cross-linking agents.
In view of inherent disadvantages of cross-linked elastomeric polymers as binder materials, there has been considerable interest in developing thermoplastic elastomers suitable as binders for solid, high-energy compositions. However, many thermoplastic elastomers fail to meet various requirements for propellant formulations, particularly the requirement of being processible below about 120.degree. C., it being desirable that a thermoplastic elastomeric polymer for use as a binder in a high-energy system have a melting temperature of between about 60.degree. C. and about 120.degree. C. The lower end of this range relates to the fact that the propellant composition may be subject to somewhat elevated temperatures during storage and use, and it is undesirable that significant softening of the propellant composition occur. The upper end of this range is determined by the instability, at elevated temperatures, of many components which ordinarily go into propellant compositions, particularly oxidizer particulates and energetic plasticizers. Many thermoplastic elastomers exhibit high melt viscosities which preclude high solids loading and many show considerable creep and/or shrinkage after processing. Thermoplastic elastomers (TPE's) typically obtain their thermoplastic properties from segments that form glassy domains which may contribute to physical properties adverse to their use as binders. Thermoplastic elastomers are block copolymers with the property of forming physical cross-links at predetermined temperatures. The classical TPE, e.g., Kraton, obtains this property by having the glass transition point of one component block above room temperature. At temperatures below 109.degree. C., the glassy blocks of Kraton form glassy domains and thus physically cross-link the amorphous segments. The strength of these elastomers depends upon the degree of phase separation. Thus, it remains desirable to have controlled, but significant, immiscibility between the two types of blocks, which is a function of their chemical structure and molecular weight. On the other hand, as the blocks become more immiscible, the melt viscosity increases, thus having a deleterious effect on the processability of the material. Above-mentioned U.S. Pat. No. 4,361,526 proposes a thermoplastic elastomeric binder which is a block copolymer of a diene and styrene, the styrene blocks providing a meltable crystal structure and the diene blocks imparting rubbery or elastomeric properties to the copolymer. This polymer requires processing with a solvent; solvent processing is undesirable in that the propellant cannot be cast in a conventional manner, e.g., into a rocket motor casing. Furthermore, solvent-based processing presents problems with respect to removal and recovery of solvent.
It has been proposed to produce thermoplastic elastomers having both A and B blocks, each derived from cyclic ethers, such as oxetane and oxetane derivatives and tetrahydrofuran (THF) and tetrahydrofuran derivatives. The monomer or combination of monomers of the A blocks are selected for providing a crystalline structure at usual ambient temperatures, whereas the monomer or combination of monomers of the B blocks are selected to ensure an amorphous structure at usual ambient temperatures. Such proposed thermoplastic elastomers (TPE's) include ABA triblock polymers, (AB).sub.n polymers in which A and B blocks alternate and A.sub.n B star polymers in which several A blocks are linked to a central, multi-functional B block. Such TPE's are thought to be highly suitable for use in binder systems for high-energy compositions, such as propellants, explosives, gasifiers, or the like. The A and B blocks of such polymers are mutually miscible in a melt of the polymer. The melt viscosities of such a TPE decreases rapidly as the temperature is raised above the melting point of the crystalline A blocks, contributing to its processability. Furthermore, a thermoplastic elastomer based upon crystalline domains exhibits advantageous solvent-resistance and minimal setup shrinkage. Such a TPE can be formulated to have a melting temperature which falls within a desirable 60.degree. C. to 120.degree. C. range, to be chemically stable up to 120.degree. C. and above, to have a low melt viscosity, to be compatible with existing components of high-energy compositions, to retain mechanical integrity when filled with solids up to 90 percent w/w, and to have a glass transition temperature below 20.degree. C. and even below -40.degree. C.
Two methods have been previously proposed for producing such TPE's. According to one proposed method, ABA triblock or (AB).sub.n polymers may be joined together through a block linking technique in which a linking moiety, such as phosgene or an isocyanate, is reacted with both ends of the middle (B) block and the end (A) blocks are subsequently reacted with the linking group (x). Generally the reaction is: ##STR1##
According to the other proposed method, an ABA polymer is formed by systematic monomer addition. For example, the A monomer may be reacted with an initiating adduct to form an A block by cationic polymerization and the reaction allowed to proceed until monomer A is substantially exhausted. Then the monomer or monomers of block B are added and polymerization proceeds from the active end of block A. When the monomers of block B are substantially exhausted, additional monomers of block A are added, and polymerization proceeds from the active end of block B. The reaction is represented by the equation: ##STR2## Alternatively, a difunctional initiator could be used to initiate the polymerization of the B block. When the A block is added, the polymerization would proceed from both active ends of the B block. The reaction is represented by the equation: ##STR3## By selection of appropriate block functionality or by repetition of steps, these methods are also proposed as being suitable for producing (AB).sub.n polymers and A.sub.n B star polymers.
Both of these methods of producing polyether TPE's have proven less than satisfactory. Joining of A and B blocks is found to be minimal at best by either of the two methods described above; accordingly, an improved process for producing TPE's having both crystalline A polyether blocks and amorphous B polyether blocks is desired.
In accordance with U.S. Pat. No. 4,806,613, issued to Robert Wardle, a three stage method is provided for forming thermoplastic elastomers having polyether crystalline A blocks and polyether amorphous B blocks. Each of the polyether blocks A and B are individually synthesized. The A blocks and B blocks are each separately end-capped with a diisocyanate in which one isocyanate moiety is substantially more reactive with active groups on the blocks than is the other isocyanate moiety. Finally, the end-capped blocks are mixed and reacted with a difunctional linking chemical in which each function on the linking chemical is isocyanate-reactive and sufficiently unhindered to react with a free isocyanate moiety on a capped block. However, to link difunctional A and B blocks and form exclusively ABA elastomers by controlling stoichiometry of the blocks is statistically unrealistic.
It is highly desirable that an even more effective, efficient and simpler method be available for unambiguously producing such thermoplastic ABA and A.sub.n B elastomers having polyether crystalline A blocks and polyether amorphous B blocks, which process does not require three stages, does not require the use of the difunctional linking compound of U.S. Pat. No. 4,806,613, and can produce substantially exclusively ABA or A.sub.n B elastomers.